Every cell in your body is a small factory, and factories make waste. Breathing makes carbon dioxide. Breaking down protein makes a compound called ammonia, which is poisonous. Getting rid of these things is called excretion.
Animals do it with organs built for the job. Plants have no kidneys, yet they still have to get rid of waste and hold their water balance steady. This post compares the two, and explains why the same problem gets very different answers in a fish, a bird, a desert plant and a mangrove.
What Excretion Is For
Metabolism is the sum of all the chemical reactions in a living thing. It builds what the body needs, and it leaves behind what the body cannot use. Some of that leftover is only waste. Some of it is active too far, and it becomes poison if it piles up.
So excretion has two jobs. It removes waste, and it keeps the inside of the body steady while the outside changes. That second job is called homeostasis.
Water and salt are part of the same job. The body is mostly water with salts dissolved in it, and it must stay in that range. Too little salt in the body fluid and water floods into the cells. Too much and water runs out of them. Controlling this is called osmoregulation.
Carbon dioxide leaves through the lungs. Everything else leaves in liquid, and almost all of it leaves through one of the organs below.
The four jobs a body does with fluid
- Filtration — push a thin fluid out of the blood and into a collecting tube.
- Reabsorption — take most of that fluid straight back into the blood, along with the salts and sugars the body needs.
- Secretion — move extra substances out of the blood and into the tube, including wastes that were never filtered out.
- Excretion — send what is left out of the body as urine.
The Nitrogen Waste Problem
Proteins and DNA both contain nitrogen. Break them down and you end up with ammonia. Ammonia is very soluble, so it crosses cell membranes easily, and that is exactly the problem. It follows water everywhere and it raises the pH of the cell until the cell stops working.
Every animal solves this the same way, by wrapping the ammonia in something safer before getting rid of it. The three main candidates trade off against each other. You want low toxicity, you want to spend little water, and you want to spend little energy.
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| Adenine and guanine. When their rings are broken down, the leftover piece is uric acid — Image: Andervik, public domain, via Wikimedia Commons |
| Waste | Used by | Water needed | Energy needed | How well it dissolves |
| Ammonia | Fish, aquatic invertebrates, protists | Very little | None to make | Very well |
| Urea | Mammals, adult amphibians | A lot | High | Well |
| Uric acid | Birds, insects, most reptiles | Almost none | Highest | Very poorly |
| Guanine | Spiders and other arachnids | Almost none | High | Very poorly |
Read that table as a series of swaps. Ammonia is the cheapest and the most dangerous. Urea costs energy to build but is safe to dissolve in water. Uric acid and guanine are almost insoluble, so an animal can dump them as a paste or a crystal and keep almost all of its water. Insects and birds that live in dry places pay the highest energy bill for that saving.
Ammonia, urea, and the liver
Mammals convert ammonia into urea in the liver. The reaction chain that does it is called the urea cycle. Hans Krebs and Kurt Henseleit worked it out in 1932. It was the first metabolic cycle ever discovered, five years before the better known citric acid cycle.
Once the urea is in the blood, the kidneys filter it out. Because urea dissolves well, you need a lot of water to carry it away. That is why mammals on dry land need access to free water, and why desert rodents make very concentrated urine instead.
Uric acid and the waste of a bird
Birds and most reptiles excrete nitrogen as uric acid. It barely dissolves, so it leaves the body as a white paste with almost no water lost. A bird egg full of liquid waste would be a disaster, and insoluble uric acid is part of why that trick works.
The same applies to insects, which is why you often see tiny white drops of insect waste on a leaf. In humans, uric acid is still a normal part of urine. When too much of it builds up in the blood, it forms needle-shaped crystals in the joints. That condition is called gout.
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| Water pushed out of a plant by root pressure. Droplets of insect waste can look similar, which is why the two are easy to confuse — Image: NoahElhardt, public domain, via Wikimedia Commons |
Water Balance: Osmoregulation
Water balance is a separate problem from waste removal, and it is the one that kills animals first. Body fluid has salts in it. Water moves by osmosis toward the saltier side. So the danger is always the same: water moving in or out faster than the body can correct it.
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| A marine bony fish loses water by osmosis, so it drinks seawater and pours out the extra salt — Image: Kare Kare, modified by Biezl, CC BY-SA 3.0, via Wikimedia Commons |
The two environments push in opposite directions, so the solutions have to be opposite too.
| Animal | The problem | The answer |
| Freshwater fish | Body is saltier than the water, so water floods in and salts leak out | Drink nothing. Make very dilute urine. Actively take salts back in through the gills. |
| Marine bony fish | Body is less salty than seawater, so water leaks out and salts leak in | Drink seawater constantly. Make very little urine. Dump extra salt through the gills. |
| Sharks and rays | Same outward leak as other marine fish | Keep urea in the blood to raise the salt level. Add a second waste chemical to stop the urea harming cells. |
| Terrestrial animals | Water is scarce and getting scarcer | Urea instead of ammonia, a kidney that reabsorbs heavily, and in reptiles uric acid paste. |
| Birds | No water source at all, sometimes | Uric acid paste with almost no water in it. |
| Insects | Can lose water through a thin cuticle | Malpighian tubules and uric acid, plus a waterproof outer coat. |
A freshwater fish is running a desalination plant. It must keep every scrap of salt it can get, and it must pour out a flood of dilute urine. A bony fish in the sea runs the opposite plant: it drinks continuously and excretes salt it does not need. The gills do the salt work in both cases. In fresh water they take salt out of the water. In salt water they push salt out into it, while also dumping carbon dioxide.
The animal with no organ at all
Single-celled freshwater protists have this problem and no kidneys, no gills, and no blood. They solve it with an organelle. The contractile vacuole fills with water, then contracts and squeezes the water out through the cell surface. One full cycle takes only seconds. The filling stage is called diastole, and the emptying stage is called systole.
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| A Paramecium in fresh water. Without the vacuoles pumping water out, the cell would swell and burst — Image: Hämbörger, CC BY-SA 3.0, via Wikimedia Commons |
The Excretory Organs of Animals
Bigger animals need more plumbing. Different body plans solved the same problem with different organs.
- Kidneys — the vertebrate answer. A human kidney is about 12 cm long and holds roughly 1 to 1.5 million filtering units.
- Nephridia — paired organs in worms and other invertebrates, doing much the same job as a vertebrate kidney.
- Malpighian tubules — thin tubes hanging off the gut in insects, spiders, centipedes and tardigrades. They empty into the gut rather than into a separate exit. Named after the seventeenth-century anatomist Marcello Malpighi.
- Contractile vacuoles — the single-cell solution, in freshwater protists.
- Salt glands — found in sharks and rays, seabirds, and some reptiles. Seabirds carry theirs above the eyes, so a bird that has been drinking seawater runs its beak and looks damp.
How a human kidney works
A kidney is a bundle of about a million nephrons. Each one is a tiny filter joined to a tube. About one fifth of the blood entering the kidneys is filtered out into the nephron. Then the body takes back almost all of it. Water, sodium, bicarbonate, glucose and amino acids all go back into the blood. Hydrogen, ammonium, potassium and uric acid are pushed the other way, into the tube.
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| Inside a kidney. The pale outer layer is the cortex, where the filtering happens, and the inner medulla holds the loops that pull water back — Image: Blausen.com staff, CC BY 3.0, via Wikimedia Commons |
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| One nephron. Filtration at the top, reabsorption down the sides, and secretion adding what the body wants out — Image: OpenStax College, CC BY 3.0, via Wikimedia Commons |
What the body takes back depends on hormones. If you are dehydrated, the hypothalamus releases antidiuretic hormone, which tells the collecting ducts to take back more water. You make less urine, and it is more concentrated. That single change can change urine output many times over.
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| The human urinary tract: two kidneys, two ureters, one bladder, and the urethra leading out — Image: Cenveo, CC BY 4.0, via Wikimedia Commons |
How Plants Get Rid of Waste
Plants have no kidneys, no nephridia, and no dedicated excretory organ. They do have cell walls, vacuoles, and a lot of water. They solve waste and water balance with the same structures they already use to breathe and to move water.
- Gases through stomata — oxygen from photosynthesis and water vapor from transpiration leave through the pores on the leaf, the same pores you studied in transpiration. Lenticels do the same job on stems.
- Vacuoles — the large storage vacuole in a plant cell is a warehouse. Waste compounds are moved in and can crystallise there, walled off from the working parts of the cell.
- Guttation — root pressure pushes liquid out of pores at the leaf edge, usually at night. What comes out is xylem sap, not pure water, so it carries dissolved salts and waste with it.
- Leaf fall — a deciduous plant locks its waste into the leaves and drops them. The leaf is then shed, which also gets rid of the waste.
- Heartwood — long-lived trees such as ebony move waste into the dead wood at the center, where it can never interfere with living tissue.
- Salt glands — mangroves living in salty soil move salt out through glands in their leaves, leaving visible crystals on the leaf surface.
- Into the soil — some plants, including conifers, release chemicals that suppress nearby plants. The effect is called allelopathy.
Leaf fall is disposal, not excretion
This is worth being precise about. In animals, excretion means waste leaves the body. In a plant, most of these routes only move waste to somewhere else inside the plant, or out into the soil. A leaf that falls has carried its waste to the ground rather than out of the organism. That is a real strategy, and it is how plants avoid poisoning themselves over decades, but it is closer to storage and disposal than to excretion.
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| The abscission line. Cells across this line weaken, and the leaf breaks off cleanly here instead of tearing — Image: Richard Avery, CC BY-SA 3.0, via Wikimedia Commons |
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| Heartwood is the dead inner wood. A tree can park waste there for hundreds of years without being harmed — Image: Rbreidbrown, CC BY-SA 4.0, via Wikimedia Commons |
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| A grey mangrove. It takes salt in faster than it needs, so it pushes the extra out through glands in the leaves — Image: Abu Hamas, CC BY-SA 4.0, via Wikimedia Commons |
There is a catch to the plant solution. A plant cannot simply run a pump like a kidney. Losing water through the leaf is what pulls water and nutrients up from the soil in the first place. So the plant is always balancing two jobs at once: get the water up, and do not lose too much of it. Between 97 and 99.5 percent of the water a plant takes up is lost again this way.
Key Takeaways
- Excretion removes metabolic waste. Osmoregulation keeps water and salt in the right balance. They are different jobs that often use the same organs.
- Animals excrete nitrogen as ammonia, urea, uric acid, or guanine, depending on how much water they have and how much energy they can spend.
- Ammonia is cheap and poisonous. Urea costs energy but dissolves well. Uric acid costs the most and wastes the least water.
- A freshwater fish must never drink. A marine bony fish must drink constantly.
- Kidneys filter the blood into nephrons, then take back almost everything the body needs.
- Plants have no excretory organs. They use the vacuole, stomata, guttation, leaf fall, and heartwood instead.
- Losing water through the leaf is not a waste. It is the force that pulls water and nutrients up from the soil.
Frequently Asked Questions
Is sweating excretion?
Partly. Sweat does leave the body, but its main job is cooling. It carries a little salt out too. The kidneys do most of the real waste removal, and sweating mainly gets rid of heat.
Why do we say a mammal excretes urea, not ammonia?
Because ammonia is too dangerous to keep. The liver wraps it into urea using the urea cycle. Urea dissolves in water and is almost harmless, so it can travel in the blood to the kidneys without harming anything on the way.
How do birds manage without losing water?
They excrete nitrogen as uric acid, which barely dissolves in water. It leaves as a thick paste or a dry crystal, so almost no water is wasted. That is also why the nitrogen waste in a bird egg is almost dry.
What is the difference between excretion and egestion?
Excretion is removing waste that the body made. Egestion is pushing out material that never entered the body cells, such as the undigested part of food. Faeces are mostly egestion. Urine is excretion.
Do plants really have no excretory organs?
They have no organ built only for excretion. But they do have specialized structures that remove specific things. Mangroves have salt glands. Some plants push liquid out through hydathodes at the leaf edge. Plants with cavities lined with glands can release chemicals as well.
Why is kidney failure dangerous?
Because the kidneys do far more than make urine. They also set blood pressure, control acid and base balance, and keep electrolytes in range. When they fail, all of those start to slip at the same time.
Sources: Wikipedia articles on excretion, the excretory system, osmoregulation, nitrogenous waste, ammonia, urea, uric acid, guanine, the kidney, the nephron, gills, the contractile vacuole, the Malpighian tubule, vacuoles, abscission, heartwood and allelopathy. Images: Wikimedia Commons, with authors and licenses noted in each caption.
